12. Asteroids and meteorites

Size, color and spin

The size of asteroids

Due to its small size, an asteroid remains an unresolved point of light in even the best telescopes on Earth, just like a faint star. This explains the name asteroid, which comes from a Greek word that means “starlike”. Although the name describes the visual appearance of these objects in a telescope, it is totally inappropriate to their physical nature. Using our instruments on Earth, we can determine the sizes of these objects, and they are much smaller than either a star or a major planet.

Even the biggest asteroids are smaller and less massive than the Moon. Ceres is by far the largest asteroid, having a radius of about 475 thousand meters and a mass of 1.17 x 1021 kilograms. That is about a third the radius of the Moon and only 0.016 the mass of the Moon. The brightest asteroid, Vesta, is even smaller, a little less than half the size of Ceres. There are many more small asteroids than big ones. About 1,000 asteroids are larger than 15 thousand meters in radius. Surveys of the faintest asteroids suggest that there are about half a million asteroids in the main belt larger than 1.6 thousand meters. Yet, despite their vast numbers, the total mass obtained by adding up the contributions of all asteroids, of all sizes, is far less than the mass of any major planet. The entire asteroid belt is only 0.05 the mass of the Moon and just 0.0006 the mass of the Earth.

An asteroid’s color

Because asteroids display no visible disk, Earth-based observers must infer their physical characteristics from the intensities and spectral properties of their reflected sunlight. By comparing an asteroid’s reflected light, wavelength by wavelength, with that of the incident sunlight, it is possible to deduce its surface composition. Astronomers divide the amount of incident sunlight at each wavelength by the amount of reflected sunlight at that wavelength, and the ratio tells them how much light of each color is reflected compared to any other color. Such spectral measurements have revealed the physical diversity of the asteroids, and shown that their compositional differences tend to depend on distance from the Sun.

The bulk of main-belt asteroids can be divided into two broad spectral categories, known as the S, for silicate, and C, for carbonaceous, types. The bright S-types have a reddish color and exhibit spectral dips identified with absorption by silicate minerals. They prevail in the inner part of the asteroid belt, orbiting closer to the Sun than the belt mid-point.

In contrast, the C-type asteroids are darker, bluer and richer in carbon, with relatively flat and featureless spectra at visual wavelengths. The C-type asteroids far outnumber all types, possibly composing three-fourths of the main belt. The C-type asteroid 1 Ceres is a representative example; it has a smooth visible spectrum with an infrared absorption feature attributed to water embedded in its mineral structure. The S-type asteroids probably account for up to 15 percent of all asteroids. Some of the less common M-type asteroids reflect sunlight in a way that suggests that their surfaces are composed of nickel and iron, hence the designation M for metallic for at least some of them. These objects could be the metal cores of larger parent bodies, stripped bare by collisions. The M-types are most common in the middle of the asteroid belt. Future space entrepreneurs may want to mine them for valuable metals.

There is an intriguing connection between the composition of asteroids and their distance from the Sun. The innermost asteroids, with orbits closest to the Sun, are rocky, siliceous and dry, while the outer ones are carbonaceous with water-rich, clay-like minerals. The igneous asteroids, found closer to the Sun, have fewer volatile compounds and less water, and they have been subject to greater heating. The primitive asteroids, which are located farthest from the Sun, are primarily rich in carbon and water. There is a related, progressive decrease in an asteroid’s reflecting power with increasing distance from the Sun. The brightest asteroids that reflect the most sunlight tend to lie near the inner edge of the main belt, closest to the Sun, while the most distant asteroids are, on the average, the darker ones with the lower reflecting power. The very darkest are found in the remote regions near Jupiter’s orbit. These differences in the composition and reflecting power of asteroids are probably related to conditions in the primeval solar nebula – the interstellar cloud of gas and dust from which the solar system originated. They may be a consequence of a decrease in temperature with increasing distance from the young Sun when the asteroids were formed.

The spin of an asteroid

Asteroids do not shine like a steady beacon with constant brightness. They instead reflect a varying amount of sunlight toward the Earth. The observed brightness variation, also known as a light curve, is periodic, often with two maxima and two minima. The overall repetition is due to rotation, while the double pattern of variability results from alternating side views of an asteroid’s elongated shape. When we see the biggest side of an asteroid, with the greatest area, the asteroid is brightest, while the smaller area reflects less sunlight and the asteroid is dimmer.

Almost all asteroids spin about a single axis. The period of rotation is inferred from the amount of time that it takes for the complete pattern of brightness variation to repeat itself. The rotation periods are usually between 2.4 and 24 hours, although a few of them rotate with longer periods, such as 253 Mathilde with a rotation period of 17.4 days and some have periods of only a few minutes. Frequent oblique collisions can increase the rate of rotation or decrease it, depending on whether the collision is in the direction or rotation or opposite to it.

Some asteroids are probably rotating as fast as they can. If an asteroid is not solid, and is thus bound only by its own gravity, it can only spin at a certain maximum rate before material is whirled off it. Asteroids larger than 200 meters seem to have reached this limit, for most of them do not rotate faster than once every 2.2 hours, suggesting that there is nothing stronger than gravity holding them together. If it lacks the material strength of a solid, an asteroid with a faster rotation rate, and a shorter period, will throw material off its surface and fly apart. Such an asteroid might resemble a rubble pile or gravel heap, formed after collisions have blasted a larger asteroid to bits, with the fragments reassembling into a loosely bound object with little internal cohesion. Close-up scrutiny by spacecraft has shown that large asteroids can be both rubble piles and solid rocks.

Asteroids smaller than 200 meters in diameter can rotate at faster rates, some turning once every few minutes. Their rotation is too rapid for these asteroids to consist of multiple components bound together by mutual gravitation. They must instead be rock solid. Some small asteroids rotate so swiftly that their day ends almost as soon as it begins. An example is the 30-meter asteroid 1998 KY26. Its day and night are only 5 minutes long; sunrises and sunsets on this asteroid take less than one second. By way of comparison, daylight at some places on Earth can last 12 hours or longer, and terrestrial sunrise or sunset usually takes about two minutes.